The diesters of naphthalenedicarboxylic acids are useful for preparing a variety of polymeric materials such as polyesters or polyamides. One particularly useful diester is dimethyl-2,6-naphthalenedicarboxylate (DM-2,6-NDC). DM-2,6-NDC, for example, can be condensed with ethylene glycol to form polyethylene naphthalate (PEN), a high performance polyester material. Fibers and films made from PEN have considerably improved strength and superior thermal properties relative to, for example, polyethylene terephthalate. For this reason, PEN is an exceptional material for preparing commercial articles such as thin films which can be used, for example, for the manufacture of magnetic recording tape and electronic components. Additionally, because of its superior resistance to gas diffusion, and particularly to the diffusion of carbon dioxide, oxygen and water vapor, films made from PEN are useful for manufacturing food containers, especially the so-called “hot fill” food containers. PEN is also useful for preparing high strength fibers which can be used to manufacture tire cord, for example.
In order to prepare high quality PEN suitable for commercial use, it is necessary to start with purified DM-2,6-NDC. The purified DM-2,6-NDC must be low in color, substantially free of organic and inorganic impurities, and low in particulate matter.
DM-2,6-NDC is most readily prepared by the esterification of 2,6-naphthalenedicarboxylic acid (2,6-NDA) with methanol. The 2,6-NDA is conveniently prepared by the oxidation of a 2,6-dialkyl- or 2-alkyl-6-acyl naphthalene compound using molecular oxygen and catalyzed by a catalyst comprising cobalt, manganese and bromine components. During this oxidation reaction, impurities such as 6-formyl-2-naphthoic acid (FNA), trimellitic acid (TMLA) and various brominated compounds are produced. Although in some instances it would be desirable to use 2,6-NDA directly for the preparation of PEN because of its high melting point (>300° C. with decomposition) and extremely low solubility in ordinary solvents, 2,6-NDA is extremely difficult to purify to acceptable levels by standard purification techniques such as distillation and recrystallization. These difficulties in purifying 2,6-NDA are partially overcome by converting 2,6-NDA to its dimethyl ester, DM-2,6-NDC. DM-2,6-NDC can be distilled and it can be recrystallized from solvents such as methanol or from one or more aromatic solvents. However, even though DM-2,6-NDC can be purified by treatments such as distillation or recrystallization, purifying DM-2,6-NDC to a purity acceptable for use in the aforementioned manufactured articles remains a problem in the art. For example, the FNA produced during the oxidation of dialkylnaphthalene is incorporated (as a methyl ester) into DM-2,6-NDC during the esterification of 2,6-NDA and is very difficult to remove or reduce to acceptable low levels. In particular, cobalt and manganese oxidation catalyst metals used for the preparation of 2,6-NDA are also typically carried over into the esterification reaction as impurities. This is because a certain amount of the oxidation catalyst metal is complexed tightly to TMLA and other oxidation by-products and is not removed in the oxidation mother liquor when the oxidation mother liquor is separated from the solid 2,6-NDA. Catalyst metals cause problems in the downstream operations used for purifying the DM-2,6-NDC by, for example, causing a thickening of the distillation bottoms and plugging of the distillation column. These catalyst metals must be removed prior to the distillation of DM-2,6-NDC.
Finally, particulate contamination in the DM-2,6-NDC must be eliminated or reduced to very low levels. Particulate contamination in the DM-2,6-NDC causes particulate contamination in PEN made from the DM-2,6-NDC. These particulate contaminants render the PEN unsuitable for manufacturing the thin, high-strength film used to prepare, for example, recording tape. These particulate impurities, which range in size down to below 1.5 microns, can arise from a variety of sources. For example, they may be oxidation catalyst particles. They may also be derived from filtering and drying operations where DM-2,6-NDC is dissolved in a solvent, recrystallized, separated from the recrystallization mother liquor by filtration and dried to remove excess solvent. Inevitably, a considerable amount of particulates contaminate the DM-2,6-NDC product in these processes. Regardless of the source, particulate contamination in the DM-2,6-NDC product is undesirable.
PEN or certain materials made from PEN can generate a relatively high rate of “scrap” or off-specification material. This material can be judged to be scrap for various reasons, including failure to meet color specifications, failure to meet molecular weight or intrinsic viscosity specifications, improper crystallinity, high diethylene glycol content, contamination by extraneous materials, heels from equipment drainage, and/or any other defect which would prohibit using the affected PEN in finished goods. Due to the high value of the contained naphthalate fraction in PEN, however, it is uneconomical to simply discard off-specification PEN. Thus, there is a need in the art to economically and conveniently reclaim the naphthalate value of PEN while simultaneously satisfying the stringent purity requirements of the final DM-2,6-NDC product.